When aircraft are flying at low altitude there is a significant hazard from striking birds in the air. If a bird should hit the windshield of an airplane it can penetrate the window and/or cause spalling of window fragments with a high degree of hazard to the pilots. This hazard becomes quite high for military aircraft which may be called upon to fly at high speeds at very low elevations. Commercial and private aircraft are also susceptible to this problem but usually fly at lower speeds at low elevations.
Aircraft used for some commercial purposes must be qualified for airworthiness by Federal Aviation Administration certification, and it is required for some such aircraft to qualify under FAA Part 25 which includes resistance to a standardized bird impact test.
In the standardized test the carcass of a four pound chicken is propelled against an airplane windshield at a selected velocity. In one aircraft, for example, it is desired to have the windhsield withstand such an impact of a four pound chicken at a velocity of 500 knots.
For successfully withstanding such a severe test there should be no penetration of the window by any part of the bird, that is, the bird should not penetrate in the area of impact and there should not be sufficient gap produced at the edge of the window to permit part of the bird to enter the cockpit. Further, the edge of the window should remain sufficiently intact and secured to the airframe that no large rush of air enters the cockpit. The window should also be resistant to spalling of fragments from the interior surface of the window.
Glass and stretched acrylic have not proved to be completely satisfactory materials for windows having resistance to bird impact at high velocity. Glass is heavy and it can break or spall easily. Stretched acrylic can be subject to many of the problems encountered in glass. Windows have been suggested employing polycarbonate resin sheets. Without special provisions these windows are heavy and difficulty can be encountered due to tearing of the window in the edge portions or breakage of mounting bolts so that a gap is formed between the edge of the window and the airframe in which it is mounted. In some aircraft the region most subject to failure is along the aft edge of the window near a region of impact a few inches forward of the aft edge.
It is therefore desirable to provide a window for airplanes which is capable of withstanding impact with birds at relatively high speeds.